Dimmer having a power supply monitoring circuit

Abstract

A two-wire dimmer for control of a lighting load from an alternating-current (AC) power source includes a semiconductor switch, a power supply, and a control circuit. The power supply includes an energy storage input capacitor that is able to charge only when the semiconductor switch is non-conductive. The control circuit continuously monitors the voltage on the input capacitor and automatically decreases the maximum allowable conduction time of the semiconductor switch when the voltage falls to a level that will not guarantee proper operation of the power supply. The dimmer of the present invention is able to provide the maximum possible conduction time of the semiconductor switch at high end (i.e., maximum light intensity) while simultaneously ensuring sufficient charging time for proper operation of the power supply, and hence, the dimmer.

1. A two-wire dimmer for control of a lighting load from a source of AC voltage, comprising:
 a semiconductor switch operable to be connected between said lighting load and said source of AC voltage; said semiconductor switch having a conducting state and a non-conducting state; 
 a power supply operable to draw current from said source of AC voltage when said semiconductor switch is in said non-conducting state; and 
 a control circuit operable to control said semiconductor switch into said conducting state for an on-time each half-cycle of said AC voltage; said control circuit coupled to an input of said power supply for monitoring a voltage of said power supply; 
 wherein said control circuit is operable to decrease said on-time when said input voltage of said power supply falls below a first predetermined voltage threshold. 
 
     
     
       2. The two-wire dimmer of  claim 1 , wherein said power supply has an input for receipt of an input voltage and said control circuit is coupled to said input of said power supply for monitoring said input voltage of said power supply. 
     
     
       3. The two-wire dimmer of  claim 2 , wherein said control circuit is operable to decrease said on-time by a first predetermined time interval. 
     
     
       4. The two-wire dimmer of  claim 3 , wherein said control circuit is operable to increase said on-time when said input voltage of said power supply remains above a second predetermined voltage threshold. 
     
     
       5. The two-wire dimmer of  claim 4 , wherein said control circuit is operable to increase said on-time by a second predetermined time interval. 
     
     
       6. The two-wire dimmer of  claim 5 , further comprising a resistive voltage divider for providing a scaled-down representation of said input voltage of said power supply to said control circuit. 
     
     
       7. A method for controlling the intensity of a lighting load from a source of AC voltage in a dimmer comprising a semiconductor switch coupled between said lighting load and said source, a control circuit for controlling said semiconductor switch, and a power supply for powering said control circuit; of ensuring that a power supply for powering low-voltage circuitry of a dimmer is able to properly charge, the dimmer adapted to be coupled between a source of AC voltage and a lighting load, said method comprising the steps of:
 scaling down a voltage of said power supply to produceproducing a sense voltage in response to a voltage of said power supply; 
 measuring said sense voltage; 
 comparing the said sense voltage to a first predetermined voltage threshold; and 
 changing a conduction time of said semiconductor switch dimmer based on the result of the step of comparing, such that said power supply has enough time to charge. 
 
     
     
       8. The method of claim  7  11, wherein said voltage of said power supply comprises an input voltage of said power supply. 
     
     
       9. The method of claim  8  11, wherein changing said conduction time comprises decreasing said conduction time by a first predetermined time when said sense voltage falls below said first predetermined voltage threshold. 
     
     
       10. The method of  claim 9 , further comprising the steps of:
 comparing said sense voltage to a second predetermined voltage threshold; 
 increasing said conduction time by a second predetermined time when said sense voltage rises above said second predetermined voltage threshold. 
 
     
     
       11. The method of claim 7, wherein the step of producing a sense voltage in response to a voltage of said power supply further comprises scaling down said voltage of said power supply to produce said sense voltage.